Communication System and Communication Apparatus

1. A communication system configured to carry out communication between a first communication apparatus and a second communication apparatus using a first key and a second key, wherein: the first communication apparatus comprises: a first storing section that is configured to store an expiration time of the first key stored in the first communication apparatus; a first managing section that is configured to manage expiration of the first key based on the expiration time of the first key stored in the first storing section; a first communicating section that is configured to transmit a first message including the expiration time of the first key at predetermined intervals; and the second communication apparatus comprises: a second storing section that is configured to store an expiration time of the second key stored in the second communication apparatus; a second managing section that is configured to manage expiration of the second key based on the expiration time of the second key stored in the second storing section; a second communicating section that is configured to receive the first message; and a correcting section that is configured to correct the expiration time of the second key stored in the second communication apparatus when the expiration time of the first key and the expiration time of the second key are judged to be different based on the expiration time of the first key included in the first message, and is configured not to correct the expiration time of the second key stored in the second communication apparatus when the expiration time of the first key and the expiration time of the second key are judged to be the same based on the expiration time of the first key included in the first message; and wherein: the second communicating section is further configured to transmit a second message including the expiration time of the second key having been corrected by the correcting section.

2. The communication system according to claim 1 , further comprising a key management apparatus that is configured to manage keys used in communication between the first communication apparatus and the second communication apparatus, wherein: the first storing section is further configured to store identification information of a first key in association with the expiration time of the first key; the first communicating section is configured to transmit the first message that further includes the identification information of the first key; and the second communication apparatus further comprises: a determination section that is configured to determine whether or not a key corresponding to the first key is stored in the second storing section based on the identification information of the first key included in the first message; and a key acquiring section that is configured to acquire the key corresponding to the first key from the key management apparatus when a determination result by the determination section indicates that the storing section of the second communication apparatus has not stored the key corresponding to the first key.

3. The communication system according to claim 2 , wherein: the first communicating section is configured to transmit the first message including a command signal that proposes updating the second key when the expiration time of the first key is less than a threshold value; and the second communicating section is configured to acquire a new second key from the key management apparatus based on the command signal.

4. A first communication apparatus configured to carry out communication with a second communication apparatus using a first key stored in the first communication apparatus and a second key stored in the second communication apparatus, the first communication apparatus comprising: a storing section that is configured to store an expiration time of the first key; a managing section that is configured to manage expiration of the first key based on the expiration time of the first key stored in the storing section; a communicating section that is configured to receive a second message including an expiration time of the second key from the second communication apparatus, the second key being corresponding to the first key and expiration of the second key being managed by the second communication apparatus; a correcting section that is configured to correct the expiration time of the first key stored in the storing section when the expiration time of the first key and the expiration time of the second key are judged to be different based on the expiration time of the second key included in the second message; and wherein: the communication section is further configured to transmit a reply message including the expiration time of the first key having been corrected by the correcting section.

5. The first communication apparatus according to claim 4 , wherein the communicating section is configured to transmit the first message including a command signal that proposes updating the second key when the expiration time of the first key is less than a threshold value.

6. A first communication apparatus configured to carry out communication with a second communication apparatus using a first key and a second key, the first communication apparatus comprising: a storing section that is configured to store an expiration time of the first key stored in the first communication apparatus; a managing section that is configured to manage expiration of the first key based on the expiration time of the first key stored in the storing section; a receiving section that is configured to receive a second message including an expiration time of the second key, the second key being corresponding to the first key and stored in the second communication apparatus, and expiration of the second key being managed by the second communication apparatus; and a communicating section that is configured to transmit a first message including the expiration time of the first key in response to the second message.

7. The first communication apparatus according to claim 6 , wherein: the receiving section is configured to receive the second message that further includes identification information of the second key; and the first communication apparatus further comprising: a determination section that is configured to determine whether or not a key corresponding to the second key is stored in the storing section based on the identification information of the second key included in the second message; and a key acquiring section that is configured to acquire the key corresponding to the second key from a key management apparatus when a determination result by the determination section indicates that the storing section has not stored the key corresponding to the second key.

8. The first communication apparatus according to claim 7 , wherein: the receiving section is configured to receive the second message including a command signal that proposes updating the first key when the expiration time of the second key is less than a threshold value; and the key acquiring section is configured to acquire a new first key from the key management apparatus based on the command signal.